A liquid crystal projector is a video equipment in which a light emitted from a light source is transmitted through an optical system (homogenizer) such as a rod lens or an array lens for homogenizing light distribution and then illuminated at the back of an image forming liquid crystal panel thereby reflecting imaging video images on a forward screen. A DLP projector is a video equipment of a type for projecting video images by utilizing a reflection type optical element referred to as a DMD device (Digital Micromirror Device) instead of the liquid crystal panel.
FIG. 14 shows a fundamental constitution of an existent light device 61 which is disposed as a light source in an equipment such as a liquid crystal projector, and it has a high-pressure discharge lamp 62 and a concave reflection mirror 63 comprising an ellipsoidal mirror or a parabolic mirror for reflecting the light.
In the high-pressure discharge lamp 62, a pair of electrode assemblies 67 are inserted in an arc tube 56 having seal portions 65A and 65B formed on both forward and backward axial ends while interposing a discharge bulb 64 therebetween through the seal portions 65A and 65B thereof.
The electrode assembly 67 is formed by welding an electrode top end 68 comprising tungsten, a molybdenum foil 69, and a molybdenum wire 70 in series, and the seal portions 65A and 65B are sealed air tightly in a state that the discharge electrode tops 68 are opposed each other in the discharge bulb 64.
Then, the tube axis ZP of the high-pressure discharge lamp 62 and the optical axis ZL of the concave reflection mirror 63 are arranged coaxially with a seal portion 65A of the high-pressure discharge lamp 62 on the side of the opening 53a of the concave reflection mirror 63 and the other sealing portion 65B on the side of the bottom 63b of the concave reflection mirror 63.
Thus, a light emitted from the discharge bulb 64 to the periphery thereof in forward and backward directions within a predetermined range of angle is reflected at the concave reflection mirror 63 and collected and illuminated to a light collection area SP of a predetermined size such as a light incident surface of a light distribution homogenizing optical system 71, for example, a rod lens disposed forward of the lamp.
In this case, when a relatively large size of a reflection mirror 63 is used as shown in FIG. 15(a), most of lights emitted for a range of angle θ21 can be utilized effectively. However, in a case of reducing the size of a concave reflection mirror 63 as shown in FIG. 15(b) along with a demand for reducing size and weight of the device, the light utilizing efficiency is inevitably lowered.
When the size of the concave reflection mirror 63 is made smaller, among the lights emitted from the discharge bulb 64 for a predetermined angle θ21, only the light emitted backward for a predetermined angle θ22 is reflected at the concave reflection mirror 63 and reaches the light collection area SP, whereas the light emitted forward for a predetermined angle θ23 leaks to the periphery and does not reach the light collection area SP. As a result, this causes a problem that not only the light utilizing efficiency is lowered but also the light is illuminated to casing parts, etc. in the liquid crystal projector equipment to deteriorate, break or denature them.
FIG. 16 is a graph showing a light distribution relating to the light emitting direction. The abscissa shows the direction of an tube axis ZL of a high-pressure discharge lamp 62, the ordinate represents the direction passing a light emission point on the tube axis ZL and crossing the tube axis ZL at a right angle, and the coaxial scales represent the ratio of the amount of light assuming that it is 100% in the direction of the ordinate.
It can be seen that what is utilized effectively among the lights emitted from the high-pressure discharge lamp 62 is only the light emitted backward for a predetermined angle θ22 (82 to 145°) and the light emitted for an angle θ23 (45 to 82°) in an amount of light from 60 to 100% is not utilized at all.
Accordingly, as shown in FIG. 17(a), it has been proposed to dispose an auxiliary mirror 72 or reflection film (not illustrated) for reflecting the light from the discharge bulb 64 of the high-pressure discharge lamp 62 to the frontal opening portion 63a of the concave reflection mirror 63 to the center of the discharge bulb 64 (light emitting point) (refer to Patent Documents 1, 2, 3, and 4).
According to this constitution, a light emitted backward for a predetermined angle θ24 is reflected at the concave reflection mirror 63 and reaches a light collection area SP, while a light emitted forward for a predetermined angle θ25 is reflected at the auxiliary reflection mirror 72 and again passed through the center of the discharge bulb 64 (light emission point), reflected at the reflection mirror 63 on the backward side and reaches the light collection area. Accordingly, leakage of light illuminated forward can be suppressed and the light utilizing efficiency is also high.    [Patent document 1] JP-A No. 2005-309372    [Patent document 2] JP No. 3184404    [Patent document 3] JP No. 3204733    [Patent document 4] JP-T No. 2005-505909
However, since the auxiliary reflection mirror 72 or reflection film reflects the light emitted from the discharge bulb 64 to the discharge bulb 64, the electrode disposed in the discharge bulb 64 is overheated by the reflection light and the amount of the electrode material to be evaporated and scattered from the top end thereof is increased, which is deposited to the inner surface of the discharge bulb 64 to possibly cause early blackening. At the same time, the temperature at the inner surface of the discharge bulb 64 on the side of the seal 65A is remarkably elevated due to the heat radiated from the electrode top that is at the height temperature during lighting of the lamp or due to the heat transmitted from the portion to possibly bulge or burst the discharge bulb 64.
Further, as shown in FIG. 17(b), it is also possible to improve the light utilizing efficiency of the high-pressure discharge lamp device 71 by providing an auxiliary reflection mirror 73 that does not reflect the light emitted from the discharge bulb 64 of the high-pressure discharge lamp 62 to the front opening 53a of the concave reflection mirror 63 but directly reflect the light forwardly (refer to Patent Document 5).
Also in this case, the light emitted backward for a predetermined angle θ26 is reflected at the concave reflection mirror 63 and reaches the light collection area SP, whereas the light emitted forward for a predetermined angle θ27 is reflected at the auxiliary reflection mirror 73 and reaches the light collection area SP. Accordingly, leakage of the light emitted forward can be suppressed and the light utilizing efficiency is also high.
[Patent Document 5] JP-A No. 2001-125197
However, since the reflection film of the auxiliary reflection mirror 73 is generally formed by stacking thin dielectric films by several tens layer or more, production is troublesome and time consuming to increase the manufacture cost, as well as it results in a problem in view of the durability such as degradation and peeling of the reflection film.
Further, the auxiliary reflection mirror 73 has to be supported by a spoke 74 made of metal. Accordingly, when the lamp 62 is lit, shadow of the spoke 74 is projected, light distribution is lost due to distortion of the spoke 74 or the spoke is oxidized and rusted due to overheating.